1. Field of the Invention
This invention relates to a motor-driven film rewind device for a camera and more particularly to a device having a timer circuit which detects the end of a film rewinding operation and automatically brings the latter to an end.
2. Description of the Prior Art
Conventional film rewinding devices for cameras includes the kind which increase the content of a film counter according to the quantity of film taken up and decrease the content according to the quantity of film rewound during a film rewinding operation. The film rewinding operation is automatically brought to a stop when the content of the film counter reaches a predetermined value.
Generally, in loading a camera with a film, the perforations provided in the film engage a sprocket and one end of the film is wound round a spool. Then, about two frames of film are fed in a blank state before the back cover of the camera is closed. In this instance, the film counter becomes operative when the back cover is closed. In rewinding the film, therefore, a portion of the film still remains on the spool even when the rewinding operation comes to a stop with the content of the film counter having been reduced to a predetermined value by a substraction process. In this condition, the film cannot be readily taken out the camera.
In one known system a switch at a film feeding rail shifts from one condition to another according to the presence or absence of the film. In rewinding the film, a change in the position of this switch enables one to detect that the film with its leader is completely rewound into a cassette, so that the film rewind operation can be ended. However, the switch increases the cost of the device. Besides, it is difficult adequately to ensure the durability and reliability of the switch. In addition to these problems, the switch tends to be exposed to the adverse effect of dust or the like each time back cover of the camera is opened and thus has not been often used in cameras which attach great importance to reliability.
To solve these problems, another system uses a timer to measure the time required to move the leader of the film from the spool or sprocket in the film cassette. It also employs some means to detect the rotation of a member which responds to the device taking up one frame of the film at a time, i.e. a member which shifts its position every time one frame of film is taken up, such as the spool or sprocket which moves in response to a film winding operation or some other member that rotates with movement of the film. The timer begins counting when the spool or sprocket rotation detecting means no longer detects the rotation of the spool or sprocket during a film rewinding operation, and upon completion of the time count stops the film rewind operation.
However, even such a film rewind device presents problems. In some cases, the film is not tightly wound on a spool disposed within a cartridge, in a loosely wound or slackly coiled state. If the camera is loaded with such a slack coil of film and the film is rewound after exposing less than than total number of frames, and particularly only a few frames, the first few moments of the film rewind operation involves merely tightening or cancelling out the slackly coiled portion of film remaining inside the cartridge while the portion of film outside of the cartridge is not actually rewound at all. However, despite this condition, the timer begins to count time. Therefore, the time count operation of the timer tends to stop and automatically bring the film rewinding operation to an end before the film portion outside of the cartridge is completely rewound. The probability of such a slackly coiled state of film is particularly high with the film cartridges for 12 or 24 frame exposures.
Conceivable solutions of this problem include sufficiently lengthening the time counting period of the timer. In the case of a film which is not slackly coiled, however, that method results in a wasteful operation of a motor in terms of time and electrical energy and degradation of operability of the camera due to the time wasted and the noise resulting from the wasteful operation.